The investigation into the fire that affected a Japanese Airlines Boeing 787 Dreamliner at Boston Logan Airport on January 7 has pinpointed the source of the fire. According to the NTSB, the JAL lithium-ion battery comprised of eight individual cells showed multiple signs of short-circuiting leading to a thermal runaway condition.

That thermal runaway condition then cascaded to other cells in the battery leading to the blaze. According to the NTSB, charred battery components indicated that the temperature inside the battery case exceeded 500°F. The focus of the investigation moving forward will now be on the design and certification requirements for the battery system.

"U.S. airlines carry about two million people through the skies safely every day, which has been achieved in large part through design redundancy and layers of defense," said Hersman. "Our task now is to see if enough - and appropriate - layers of defense and adequate checks were built into the design, certification and manufacturing of this battery."

Boeing 787 production line [Image Source: Boeing]

The investigation has ruled out mechanical impact damage to the battery and external short-circuiting. There were signs of deformation and electrical arcing on the battery case not related to the cause of the fire according to investigators. Boeing had tested the battery during the 787 certification process and found no evidence to support that this sort of fire within the battery pack could occur.

Boeing has issued a statement on the investigation update stating that it plans to remain committed to working with the NTSB and the FAA along with its customers to maintain a high level of safety. “The 787 was certified following a rigorous Boeing test program and an extensive certification program conducted by the FAA. We provided testing and analysis in support of the requirements of the FAA special conditions associated with the use of lithium ion batteries,” said Boeing’s Marc Birtel. “We are working collaboratively to address questions about our testing and compliance with certification standards, and we will not hesitate to make changes that lead to improved testing processes and products.”

quote: According to the NTSB, the JAL lithium-ion battery comprised of eight individual cells showed multiple signs of short-circuiting leading to a thermal runaway condition.That thermal runaway condition then cascaded to other cells in the battery leading to the blaze.

I think the investigators have got the wrong end of the stick. They are saying the battery cells are the problem, not the environment, but I think the environment is the problem, not the battery, or rather, the battery isn't the right one for the environment.As I understand it, this battery has a nominal voltage of 30 volts, and each cell has a nominal voltage of 3.7 volts, meaning the entire battery consists of just 8 cells, so all cells in the battery showed the same signs failure.According to one media report, Japanese Transport Safety Board investigator Hideyo Kosugi said "I'm sure that too much current or too-high voltage has gone to the battery".(Link removed to avoid spam detection)According to the Yuasa spec sheet for this battery, the maximum "operative ambient temperature" for each cell is 65 deg C. (See their spec sheet here: (link removed to avoid spam detection))As I see it, as the internal temperature goes up, so the resistance of the internal insulation drops, thus the internal leakage current increases within each cell. Normally the heat generated by the internal leakage is so small that the battery as a whole can dissipate it happily, and there isn't a problem.If, however, due to some environmental factors the battery cells aren't able to dissipate the internal heat as fast as it is generated, e.g. an alternating high discharge current and then a high charge voltage are applied to the battery, or the battery cells are tightly packed in a poorly ventilated area, so the internal temperature rises above 65 deg C, then you could one of those endless loop problems.For example, say a sustained cycle of load and charging is placed on the battery and the raises the internal temperature rises above 65 deg C, so the leakage current also rises to the point the heat generated by it is faster than the battery can dissipate, then you would have a cascading effect, where the rising temperature allows more the leakage current, and the increasing leakage current raises the internal temperature, and you would get an almost impossible to stop situation.It may well be this battery could, in just seconds, go from being "a bit hot" to erupting like a volcano.